Peter M. Struk and Timothy M. Bencic

Glenn Research Center, Cleveland, Ohio

Jen-Ching Tsao

Ohio Aerospace Institute, Brook Park, Ohio

Dan Fulekand Daniel C. Knezevici

National Research Council of Canada, Ottawa, Ontario, Canada

Preparation for Scaling Studies of Ice-Crystal Icing 

at the NRC Research Altitude Test Facility

NASA/TM—2013-216571

September 2013

AIAA–2013–2675
	
	
	








	



	

!

	
	
#$#	




		


	
	

		


#>
	

@	



	Q	

		


X	
	
	



	
\

	_
	

			


	




	
`
	

_	

{

#_
		`



`	
	#|

} >>>@_
	




		
	

	 


				


	
	

	
	

#






	







		


		
	{	`
	


	

		





	


		


} >>@_\ 

		
	#
	 


Q	$	 

	
	`
	$	
	






\




#

} >@_>@__@_

#{

	 


	
		

} >@_>>@>


		




	#


	
	
	




	
	


	#

} >>@


	
	
	

	

 

	
	
	






	`

	

} >>_@{

	


	


		
X



Q	
	



	

Q
	Q

			


	

	
	




	
	


`|

} 	
	




http://www.sti.nasa.gov

} {
#
	

help@sti.nasa.gov

} #
	




	

\$

} 

	

\$

 

} 	
|


	

\$

 >	
		

	



	\	


	\
Peter M. Struk and Timothy M. Bencic

Glenn Research Center, Cleveland, Ohio

Jen-Ching Tsao

Ohio Aerospace Institute, Brook Park, Ohio

Dan Fulekand Daniel C. Knezevici

National Research Council of Canada, Ottawa, Ontario, Canada

Preparation for Scaling Studies of Ice-Crystal Icing 

at the NRC Research Altitude Test Facility

NASA/TM—2013-216571

September 2013

AIAA–2013–2675


	



	


_	>	 

>@


		
	



		

>
	



	#
	
	
	


\
>
	
Acknowledgments

`
	$`	
	
		
	
`
_	>

>


	`
$
`
	
	`
	$#

		

#

!

	
	X
#	
	

_	>
	

	
	
	

>$	
\	_
>


	

	



		
	


`
	$#
	`
$
$	>	#
	
`
	$		


	



>	
		

	



	\	


	\



	

	

`_


	

	
#|```


	
	
	$		
	
	



#	




	


		
	
#
	



	


Level of Review|
	#	`#


NASA/TM—2013-216571 1 

Preparation for Scaling Studies of Ice-Crystal Icing 

at the NRC Research Altitude Test Facility 

Peter M. Struk and Timothy M. Bencic 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Jen-Ching Tsao 

Ohio Aerospace Institute 

Brook Park, Ohio 44142 

Dan Fulekand Daniel C. Knezevici 

National Research Council of Canada 

Ottawa, Ontario, Canada K1A 0R6 

Abstract 

This paper describes preparation for ice-crystal icing scaling work utilizing the Cascade rig at the 

National Research Council (NRC) of Canada’s Research Altitude Test Facility (RATFac). Tests 

supporting this work and continuing the collaboration between National Aeronautics and Space 

Administration (NASA) and NRC on ice-crystal icing took place between March 26 and April 11, 2012. 

The focus was on several aspects but emphasized characterization of the RATFac cloud including watercontent and test-section uniformity as well as particle-size measurements. Water content measurements 

utilized the Science Engineering Associates (SEA) Multi-Element probe while cloud uniformity 

measurements used light scattering from particles passing through a laser sheet. Finally, particle sizespectra measurements used two developmental shadowgraph systems. Details of these measurements as 

well as selected results are presented. An analysis algorithm is presented that interprets mixed-phase 

measurements from the SEA probe using calibrations from individual water and ice clouds. The analysis 

is applied to one mixed-phase data set generated with a glaciated cloud combined with supplemental 

water. The test section temperature was below freezing to prevent the natural melting of the ice crystals. 

The analysis algorithm relies on the measurement of test-section humidity to account for cloud 

evaporation. Results of the cloud-uniformity measurements using scattered light suggest that the 

measured intensity is a good first-order measurement of concentration, independent of the water phase. 

Steeper intensity gradients across the test section are observed with increasing ice-water content. For 

particle-size measurements, both shadowgraphy methods provide high-quality images of the particles. 

These images will be processed to establish particle-size distributions and morphology characteristics. 

The results from this work will help guide future ice-crystal icing research including scaling studies. 

Nomenclature 

AC cross sectional area of test section (0.0337 m2) 

DV10, DV50, DV90 particle diameter below which the cumulative particle vol. is 10, 50, or 90 percent of 

the total sample vol. 

FOV field of view 

fps frames per second 

IWC ice water content (g/m3

) 

LED light emitting diode 

LWC liquid water content (g/m3

) 
NASA/TM—2013-216571 2 

m mass flow rate of supplemental liquid water (g/s) 

m, mass flow rate of air from cold jet which delivers ice crystals (g/s) 

m,	 mass flow rate of air entrained from surroundings of the warm side of RATFac into 

the test section (g/s) 

m mass feed rate of ice (g/s) 

MMD mass median diameter (m) 

MVD median volumetric diameter (m), used interchangeably with DV50 

P0 total pressure (kPa) 

PIV particle imaging velocimetry 

q

 = V
  A volumetric flow rate of air at test section (m3

/s) 

RATFac Research Altitude Test Facility 

RH relative humidity ( percent) 

ROI region of interest 

SEA Science Engineering Associates, Inc. 

SH, SHoff, SHon specific humidity (gH2O/kgdry air), on/off denotes measurement with cloud on/off, 

respectively. 

TAT total air temperature (C) 

TWC total water content (g/m3

) TWC  LWC + IWC 

V test-section centerline velocity (m/s) 


 air density (kg/m3

) 

Subscripts and Superscripts for LWC, IWC, or TWC 

e evaporated 

FR false response on LWC elements due to ice crystals 

i injected 

m measured 

s sublimated 

t test section 

2 2.1 mm diameter element 

0.5 0.5 mm diameter element 

* calculated value using the mixed-phase hypothesis 

1.0 Introduction 

Due to numerous engine power-loss events associated with high-altitude convective weather, 

potential ice accretion within an engine due to ice-crystal ingestion is being investigated collaboratively 

by National Aeronautics and Space Administration (NASA) and the National Research Council (NRC) of 

Canada with additional sponsorship by both the Federal Aviation Administration and Transport Canada. 

These investigations examine the physical mechanisms of ice accretion on surfaces exposed to ice-crystal 

and mixed-phase conditions, similar to those believed to exist in core compressor regions of jet engines. 

The investigations are largely experimental using the NRC Research Altitude Test Facility (RATFac) and 

designed to test hypotheses of engine-core icing processes including particle impact, phase change, 

accretion, erosion, etc. 

Early investigations from these collaborative efforts suggest that increasing altitude (i.e., decreasing 

pressure) plays a role in ice-crystal icing as it enhances evaporative cooling and lowers the wet-bulb 

temperature (Refs. 1 and 2). It is desirable to see if equivalent icing conditions could be realized near sea-
NASA/TM—2013-216571 3 

level which can be advantageous for testing from a cost perspective as altitude facilities can be costly to 

operate. Altitude scaling (Ref. 3) for ice-crystal accretion is a condition scaling method to determine the 

scaled test conditions (icing near sea level) that produce the same non-dimensional ice accretion result as 

the reference condition (i.e., icing at altitude). To date, only limited testing has been performed to 

understand if such altitude scaling can be realized. In addition, current scaling methods for super-cooled 

water droplets need to be extended to include the ice-crystal only and mixed-phase regime. Therefore, 

further work both in terms of theoretical development and experimental validation is required. 

NASA and NRC are currently working on scaling methods for ice-crystal icing. In preparation for future 

testing at RATFac dedicated to scaling, a test entry took place at the RATFac from March 26 to April 11, 

2012. This 2012 test entry had three primary objectives broadly categorized under the following topic 

areas: (1) cloud characterization, (2) scaling model developments, and (3) ice-shape measurement. The 

emphasis of this paper is on the first objective, cloud-characterization, and includes results from water 

content, cloud uniformity, and particle size-spectra measurements. A complimentary paper in this 

conference (Ref. 4) reports some early progress by NRC on altitude scaling with ice-crystal accretion. 

2.0 Background 

The first series of experiments by the NRC and NASA group was conducted at RATFac in 2010 

(Ref. 1). These experiments utilized a single wedge-type airfoil designed to facilitate fundamental studies 

while retaining critical features of a compressor stator blade or guide vane. The airfoil was placed in a 

small Cascade wind tunnel located within RATFac and icing tests were performed at various Mach 

numbers, altitudes, temperatures, as well as ice and liquid water contents. The ice appeared well adhered 

to the surface in tests at higher altitude (lower pressure) and, in several cases, showed continuous leadingedge ice growth to a thickness greater than 15 mm in 3 min. Such deposits were not observed in the 

lower-altitude (higher-pressure) tests, where the accretions were limited to a small area around the leading 

edge. The icing behavior at high and low pressure appeared to correlate with the wet-bulb temperature, 

which was estimated to be above 0 °C in the lower altitude tests and below 0 °C in tests at higher altitude, 

the latter enhanced by more evaporative cooling of water. 

Shortly after the 2010 test entry, the NRC facility added the capability to control the humidity in the 

wind tunnel. This allowed the humidity content to be increased from a minimum value of –50 C dew point. 

Increasing humidity also increases the air wet-bulb temperature up to the limit of 100 percent relative 

humidity (RH) in the test section when the wet bulb temperature equals the surrounding air temperature. At 

RH lower than 100 percent, the air wet-bulb temperature is less than the surrounding dry-air temperature 

due to evaporative cooling of the water. It is believed that both the droplets and regions of the airfoil can 

approach the wet-bulb temperature under certain conditions. After these initial tests, a hypothesis was posed 

that when the wet-bulb temperature is below freezing then a well-adhered type of icing can occur. 

In 2011, a second series of tests was conducted to examine the effects of wet-bulb temperature on the 

icing characteristics. A sub-set of those results was reported in 2012 (Ref. 2) which directly examined the 

wet-bulb temperature as an icing indicator. These tests independently varied temperature and humidity to 

achieve the same wet-bulb temperatures which showed that icing occurred when the wet-bulb temperature 

is below-freezing. These results confirmed that wet-bulb temperature is a good indicator of the potential 

for icing. Finally, it is important to point out that ice accumulation was observed with wet-bulb 

temperatures above freezing when the ice concentrations were quite high. 

Although wet-bulb temperature may be a good indicator for the icing potential, other well-established 

factors also influence icing. These include factors such as the droplet collection efficiency (governed by 

geometry, flowfield, particle size, etc.) and water loading and state. Regarding the water state, all 

previous studies show that the presence of liquid water is required for ice accretion to occur. Furthermore, 

Mason et al. (Ref. 5) hypothesized that there is an optimum ratio of water to ice that maximizes ice 

accretion for a given water loading. Experimentally, the liquid water can come from the melting of ice 

crystals or be added separately. These other factors are considered in the present studies, as well. 
NASA/TM—2013-216571 4 

The NRC RATFac uses a hot-wire probe, the Science Engineering Associates (SEA) multi-wire 

probe, model WCM-2000, to determine both the liquid and total water content. The probe has three 

elements: one element (2-mm half-pipe cross-section) for measuring the total water content (TWC) and 

two additional elements (2.1- and 0.5-mm diameter cylinders) for measuring the liquid water content 

(LWC). Data from both the 2010 and 2011 tests indicate that the TWC measurement in the presence of 

ice is significantly lower than the expected TWC based on bulk mass measurements of ice injected into 

the tunnel. Also, the TWC-element response at higher injected IWC showed a periodic response which 

was not understood at the time of the testing but was later confirmed with the manufacturer to be related 

to WCM-2000 controller settings. To date, the LWC elements have only been calibrated for use in pure 

liquid conditions and it is not clear if the LWC measurement in mixed-phase condition accurately 

represents liquid water content of the mixed phase. The 2012 test entry dedicated a significant portion of 

the test time to begin characterizing the SEA Multi-Element probe in mixed-phased conditions. 

For experiments in RATFac, the typically reported IWC and LWC values are bulk-averaged values 

and are calibrated using the SEA Multi-Element probe prior to a test. These values, subsequently called 

IWCi or LWCi in this paper where the subscript denotes the injected bulk value, are calculated by 

dividing the mass feed rate (set point) of ice and/or water by the volumetric flow rate of air, q

, through 

the test section. The q

 is calculated by taking the measured velocity at the centerline of the test section 

(under cloud-off conditions) and multiplying by the cross-sectional area of the test section. 

The uniformity of the ice or water distribution across the test section is important to quantify for all test 

conditions. NRC has previously reported some uniformity results using a process in which they traverse the 

SEA Multi-Element probe across the test section. These results provided several discrete point 

measurements across the test section. To supplement those results, NASA examined another technique 

during the 2012 test entry which utilized a laser-sheet positioned across the test section to provide additional 

uniformity information. As particles passed through the laser sheet, some light would be forward-scattered 

and would be detected by a downstream imager (not in the flow). This technique was previously used at the 

NASA Icing Research Tunnel (Ref. 6) to help visualize the uniformity of a liquid-water cloud. 

During the 2011 test entry, the RATFac included a new probe capable of measuring total air 

temperature (TAT) and humidity at the test section in both dry and wet conditions. This probe, hereafter 

referred to as the TAT-RH probe, was mounted approximately 51 mm from the base of the tunnel and in 

the same plane as the SEA Multi-Element probe. During testing, measurements from the TAT-RH probe 

indicated a drop in stagnation air temperature and an increase in test section humidity as the cloud was 

turned on. This suggests that energy is transferred from the air to the cloud causing ice particles to melt 

and/or sublimate, as well as liquid-water evaporation. This significant observation implies that the dry 

conditions, measured prior to the start of an icing test, do not accurately represent the conditions near the 

test article while the cloud is on. Later in 2011, NRC added the capability to traverse the TAT-RH probe 

vertically along the centerline of the test section. In the current 2012 test campaign, measurements at 

various locations in the flow were made by the TAT-RH probe as well as the existing total pressure and 

temperature probe (P0 and TAT probe) which can only be used in dry conditions. Data comparing the 

TAT-RH and P0 and TAT probe measurements is available but is not presented in this paper. 

Another key parameter in studying icing phenomenon is the particle-size distribution. At RATFac, an 

ice-particle cloud is generated by a grinding system with different configurations to vary the median mass 

diameter (MMD) of the cloud. A shadowgraph particle sizing method using PIV hardware (Ref. 7) is 

currently being used by NRC to determine the size distribution of various clouds. Preliminary results 

suggest that the “small” grinder configuration yielded a cloud MMD of 50 to 
`large” 

configuration generated an MMD of 200 to 
 (Ref. 8). This early work used measurements 

upstream of the bellmouth. During the 2012 test entry, the NRC PIV system was setup to measure particle 

sizes at the test section for the first time. Furthermore, NASA experimented with a similar shadowgraph 

technique using a high-speed camera coupled with a pulsed light-emitting diode (LED) which measured 

particles just downstream of the ice-ejector duct. This offered the possibility to discriminate particle phase 

by making two sets of measurements at different downstream locations. 
NASA/TM—2013-216571 5 

3.0 Cloud Characterization 

During testing, ice particles, supplemental liquid water, or both are injected into the Cascade test rig. A 

schematic representation of the test setup is shown in Figure 1. The injected mass feed rates of the ice and 

flow rates of supplemental liquid water content are denoted as m, m, respectively. The supplemental liquid 

mass can be injected using up to 8 nozzles. For this testing, typically 2 or 4 nozzles were used. The air mass 

flow at the test section is drawn by downstream compressors and is comprised of air from a cold jet, m,, 

with the remainder of the air being entrained from the surroundings, m,	, on the warm side of the 

facility. The humidity at the test section can be varied by changing the humidity of the entrained air and can 

range from a minimum value of –50 °C dew point to higher values using a humidity injection system. The 

RATFac measures specific humidity, SH, at the test section continually both with the cloud off and on. The 

humidity at the test section increases when the cloud is turned on indicating that sublimation or evaporation 

is occurring as the ice and water particles traverse from the injectors to the nozzles. 

An important technical requirement for this research is being able to quantify the cloud conditions at 

the test section based on injected values for different flow and ambient conditions. The mass 

concentrations of ice and supplemental liquid water at the test section are denoted with the subscript t 

(e.g., IWCt and LWCt). For clouds with just water sprays, the LWCt can be calculated from the injected 

mass flow rates and specific humidity measurements as shown in Equation (1). Here, LWCi is equal to the 

injected m divided by the volumetric flow rate of air, q

, at the test section, as shown in Equation (2). 

The LWCi is considered the test-section average value assuming no evaporation. The mass concentration 

of water vapor gained at the test section due to evaporation is denoted as LWCt,e as calculated in 

Equation (3). Note that Equations (1) to (3) assume uniform mixing at the test section and other modes of 

mass loss (e.g., splashing/accretion on the windows or other areas of the facility upstream of the test 

section) are negligibly small. 

LWC = LWC  LWC,	 (1) 

LWC = 







 (2) 

LWC,	 = 
(SH  SH) (3) 

Figure 1.—Schematic representation of the icing test configuration at RATFac where the Cascade rig is placed 

in the warm section and the test-section flow is drawn by downstream compressors (not shown) and is 

comprised of a cold jet mixed with warm entrained air. 

Cold Jet 

Warm Entrained Air 

Test Section 

RH% 

Laser sheet 

(See Section 3.2.)



Bellmouth
NASA/TM—2013-216571 6 

For fully-glaciated clouds at temperatures below freezing, the values for IWCt can be calculated 

similarly to the liquid-water-only sprays as shown in Equation (4). Here, IWCi is the average ice-particlemass concentration at the test section assuming no sublimation, as shown in Equation (5), and IWCt,s is 

the vapor-mass concentration gained due to ice particle sublimation from injection to the test section, as 

shown in Equation (6). In this case, ice is assumed only to sublimate due to the cold conditions. Again, 

uniform mixing at the test section is assumed. 

IWC = IWC  IWC, (4) 

IWC = 







 (5) 

IWC, = 
(SH  SH) (6) 

Finally, if both ice and supplemental water are injected, then the TWCt at the test section, as shown in 

Equation (7), can be calculated from the injected total water content, TWCi as calculated by Equation (8), 

and subtracting the amount of mass concentration (assuming uniform mixing) that sublimated or 

evaporated, TWCt,s+e as calculated by Equation (9). Here, TWCi is the combined ice and liquid water 

mass-concentration averaged across the test section assuming no sublimation and evaporation. The vapormass concentration gain at the test section, TWCt,s+e, comes from both sublimation and evaporation as the 

ice and liquid water flow, respectively. Note that the portion of the vapor increase due to ice sublimation 

versus liquid water evaporation cannot be separately measured in this mixed-phase cloud. However, a 

later section will describe an algorithm to deduce the individual IWCt and LWCt at the test section under 

mixed phase conditions. 

TWC = TWC  TWC,	 (7) 

TWC = 







 (8) 

TWC,	 = 
(SH  SH) (9) 

The aspects examined in this study include (A) measurements of LWC and TWC, (B) cloud uniformity, 

and (C) particle spectra. 

3.1 SEA Multi-Element Probe Measurements 

The total and liquid water contents at the test section are measured using the SEA multi-wire probe 

(Ref. 9). The TWC is measured using a hot-wire element with a half-pipe cross section where the concave 

portion is facing the incoming flow. A basic premise for the half-pipe element is that it captures and 

completely vaporizes all of the incoming ice and water particles while maintaining the element at a 

prescribed temperature of 140 °C. The power required to maintain the element at 140 °C is related to the 

impinging water and ice. While the multi-wire half-pipe measurement corresponds well with blade 

measurements in Appendix C supercooled liquid (Ref. 10), previous studies in ice-crystals (Ref. 11) have 

measured values lower than the expected values of TWC. One factor is that the conversion from power to 

TWC does not account for the heat of fusion for ice. However, this factor is small and would lower the 

calculated TWC for a given power measurement. Rather, it is speculated that the under reading in ice 

crystals is due to ice-particle splashing, bouncing, or otherwise flowing away prior to being vaporized by 

the element. Sections below further explore the TWC measurement in ice crystals presenting some 

quantitative results and probe imaging. 

The LWC is measured by two heated elements with a cylindrical cross section of 0.5 and 2.1 mm in 

diameter. Measurements from those elements are denoted as LWCm,0.5 and LWCm,2, respectively. The 

power required to maintain the element at 140 °C is related to the impinging liquid water assuming that 
NASA/TM—2013-216571 7 

all LWC that contacts the wire evaporates. Data from the development of the SEA multi-wire probe 

(Ref. 9) suggests that the different diameter cylinders, just as with the TWC measurement, do not respond 

identically to liquid clouds of different MVD. In fact, the original intent of the multiple-diameter 

cylinders was to discriminate between clouds of different droplet sizes owing to the different collection 

efficiencies of the cylindrical wires. In pure ice-crystal conditions, both manufacturer testing as reported 

in the owner’s manual and an internal NASA study (Ref. 12) have indicated that the LWC elements have 

a false response to the presence of ice crystals. This is expected as some of the impinging ice would melt 

and cool the heated wire. In mixed phase, it is not known if the LWC elements will accurately 

differentiate between liquid and ice. 

During the 2012 test entry, a number of tests examined the SEA multi-wire probe measurements to 

pure IWC and LWC sprays as well as combined IWC and supplemental LWC sprays. The ice-particle 

clouds used the “small” size ice grinder configuration of the RATFac (Ref. 8). The tests were conducted 

at a total air temperature below freezing of approximately –1 C to eliminate melting of the ice-particle 

cloud. Data from tests conducted at test-section speeds of V ~ 85 m/s and a total pressure of P0 ~ 93 kPa 

are presented in this paper as they represent a complete dataset from this test entry. Both the velocity and 

pressure were measured at the centerline of the tunnel just prior to the cloud on condition for nominally 

30 sec and averaged. Slight variations in the airspeed of 0.4 m/s (standard deviation) were observed 

during a given test with the mean values over the dataset varying 1 m/s (standard deviation). 

Sample results from a single mixed-phase test are shown in Figure 2 with the test conditions listed in 

the caption. The top graph of this figure shows the raw readings from the 3 elements of the SEA multiwire probe as a function of time. As the cloud comes on, the readings from the cylindrical elements (0.5 

and 2.1 mm) stabilize in ~10 sec with the half-pipe taking slightly longer at ~20 sec; these transient times 

are a combination of the cloud ramp-up time and/or probe response time. The cloud remains on for about 

3 min with the last 90 data points (~100 sec) averaged to represent the reading for that test. For the data 

shown in Figure 2, the resulting average values are 5.23  0.38, 1.51  0.06, and 0.80  0.03 g/m3

 for the 

TWCm, LWCm,0.5, and LWCm,2, respectively, where the error bars are the standard deviation of the 

measurement during the 100-sec period. The averaged values are superimposed as horizontal hatched 

lines on the top graphs of Figure 2. All cases compared in this study are analyzed in an identical way with 

the 90-point average values compared in the subsequent analysis. The measured values listed in 

subsequent data tables also include the standard deviation (shown as error bars1

) of the measurements 

during the 100-sec sampling interval. For some limited cases, there were unexplained spikes or rapid 

transient behaviors which were not immediately understood. For those data points, the time period for the 

average values was shifted to exclude those transient portions. A carrot (^) is included next to the data 

point number in the data tables below if the time period for the average values was shifted. In no 

circumstances did these data points appear to be unusable. 

The bottom graph of Figure 2 shows the raw readings of the SH at the test section measured 

approximately 89 mm below the centerline of the tunnel which corresponds to 36 mm above the bottom 

wall. The RH values are calculated based on these SH measurements as well as the instantaneous local 

total temperature at this below-centerline location. SH measurements from this below-centerline location 

were typically higher by 0.04  0.04 gH2O/kgdry air compared to the centerline location. Here the error bar 

represents the standard deviation of the SH variation for the 85 m/s and 93 kPa dataset. For data point 673 

shown in Figure 2, the SH reading increased by roughly 0.5 gH2O/kgdry air after approximately 60 sec of 

spray time. Correspondingly, the relative humidity at the test section increased although it appears to 

increase at a slightly faster rate than the SH. This behavior is likely attributable to the different 

measurement response times for SH and temperature as well as the time it takes for conditions to stabilize 

from the cloud-off to cloud-on conditions. 

 

1

 For individual measurements, the error bar represents the standard deviation of the measurements from the mean 

value of that measurement during the sampling period (typically 100 sec or as noted). For data sets, the error bars 

represent the standard deviation of the mean calculated from all data points in that data set. 
NASA/TM—2013-216571 8 

Figure 2.—Test data sample showing raw readings from the 3 elements of the SEA multi-wire probe 

(top graph) and test section humidity measurements (bottom graph). This data is from a mixedphase test (data point 673) where the IWCi = 9.86 g/m3

, LWCi = 0.87 g/m3. During cloud-off 

conditions, the test section total velocity, pressure, and temperature were measured to be 

86.2  0.4 m/s, 93.08  0.04 kPa, and –0.8  0.1 C, respectively. 

Figure 3.—Measured liquid water content as a function of liquid water content at the test section using a liquid-only 

spray at the following nominal conditions: TAT = –1C, V=85 m/s, and P=93 kPa. The LWCt comes from subtracting 

the amount of mass evaporated from the injected values, as shown in Equation (1). 

3.1.1 LWC-Only Results 

Results from LWC-only tests are shown in Figure 3 with the raw data from the measurements 

shown in Table 1. In this figure, the liquid water content at the test section, LWCt, is compared to 

measurements from all three elements of the hot-wire probe. In the figure, a reference line is shown 

where LWCt = LWCm allowing a quick comparison of the measured versus test-section values. Overall, 

the LWCm values agree well with LWCt. 
NASA/TM—2013-216571 9 

TABLE 1.—DATA FROM LIQUID-WATER ONLY SPRAYS 

[The test section velocity was nominally 85 m/s and the ambient pressure was 93 kPa and the total air temperature was –1 C. A 

carrot (^) is included next to the data point number when the time period for the average values was adjusted from the last 100-sec.] 

Data

Point 

Nozzles RH%

(cloud off)

LWCi LWCt,e LWCt TWCm

(half-pipe)

LWCm,2

(2 mm)

LWCm,0.5

(0.5 mm)

620 2 24.7 0.88 0.36 0.52 0.55  .08 0.53  .01 0.53  .01 

594 2 22.1 1.69 0.66 1.03 1.24  .09 1.06  .01 1.08  .02 

598 4 20.0 1.77 0.81 0.96 1.04  .08 0.93  .02 0.92  .03 

641^ 4 25.3 2.63 1.07 1.56 1.57  .09 1.36  .02 1.52  .03 

657 2 37.5 0.88 0.31 0.57 0.57  .04 0.55  .01 0.52  .01 

668 2 72.8 0.88 0.22 0.66 0.68  .04 0.61  .02 0.59  .02 

Generally, the TWCm (i.e., half-pipe) values are greater than either the LWCm,0.5 or LWCm,2 values. 

For the cases where LWCt is approximately 1 g/m3

 and less, both LWCm,0.5 and LWCm,2 produce very 

similar readings and were within 5 percent of LWCt. However, there was one case (data point 594) where 

TWCm was approximately 20 percent greater than LWCt. Further examination of data point 594 did not 

offer any apparent anomalies. 

For the larger test-section LWCt of 1.5 g/m3

 (data point 641 in Table 1), the LWCm,2 is about 

13 percent less than TWCm but LWCm,0.5 is within a few percent of LWCt. Such differences in response 

from the 0.5-mm and 2.1-mm elements have previously been attributed to drop size effects (e.g., see 

Figure 5 in Ref. 9). In the present tests, the liquid water spray is set at conditions which produced an 

MVD of ~40 m at the nozzle outlet plane based on a sea-level calibration of a single nozzle. However, 

the actual MVD at the test section is likely larger than 40 m and, in fact, may vary from test to test due 

to the varying degree of evaporation that is occurring in these tests. Data point 641 did have a small spike 

(i.e., rapid increase then decrease) near the end of the sampling period which was not included in the 

analysis. Otherwise, data point 641 did not appear anomalous. Repeating this data point is warranted as 

well as comparing to spray MVD results before any further conclusions can be drawn. 

Examination of the data (points 620, 657, and 668) in Table 1 shows that as the RH of the test section 

increases under cloud-off conditions, the evaporated water mass decreases at the test section for the same 

injected LWCi. This results in an increase in LWCt which is detected by all three elements of the probe with 

most of the readings being within 5 percent of LWCt with the remaining point being within ~10 percent of 

LWCt (7.6 percent for LWCm,2 and 10.6 percent for LWCm,0.5 in data point 668). 

3.1.2 IWC-Only Results 

Results from IWC-only tests are shown in Figure 4 and Figure 5. In Figure 4, the ice water content at 

the test section, IWCt, is compared to measurements from the half pipe, IWCm. In this figure, a reference 

line (solid black) is shown where IWCt = IWCm. The measurements are roughly half of the calculated testsection IWCt, with a linear curve fit of the data (hatched line in Figure 4) yielding a factor of 48.5 percent 

measured versus test section. This results is consistent with previously published results (Ref. 13) although 

those results are at a different velocity and suggest that the measurements effectiveness varies with airspeed. 

Figure 5 shows the measured false response, LWCFR, for both of the cylindrical elements, LWCm,0.5 and 

LWCm,2, along with a linear curve fit of the data. Again, a false response is the signal produced by ice 

crystals on the cylindrical elements which are intended for liquid water measurements. The curve fits 

suggest a false response of approximately 4 to 5 percent of the injected ice-particle clouds generated in this 

study. However, the dataset is sparse with some scatter. Furthermore, the correct relationship between the 

LWCFR and IWCt may not be linear and would require more data to better define. 

The data used for the calculations of the IWCt in Figure 4 and Figure 5 including the measured 

evaporation are shown in Table 2. Examination of the data shows that the amount of evaporated mass 

increases with injected IWCi when maintaining the same test-section RH under cloud-off conditions (data 

points 576, 582, and 588). Also, the evaporated mass decreases with increasing test section RH (data 

points 576 and 569) as expected and is similar to the LWC-only tests. 
NASA/TM—2013-216571 10 

 

Figure 4.—Measured IWC from the half-pipe versus 

test section IWCt at the following nominal 

conditions: TAT= –1 C, V=85 m/s, and P=93 kPa. 

Figure 5.—False response of the LWC elements 

versus IWCt. The IWCt comes from subtracting the 

sublimated mass from the injected values. 

TABLE 2.—DATA FROM ICE-ONLY SPRAYS 

[The test section velocity was nominally 85 m/s and the ambient pressure was 93 kPa and the total air temperature was –1 C. A 

carrot (^) is included next to the data point number when the time period for the average values was adjusted from the last 100-sec.] 

Data point RH 

(cloud off), 

% 

IWCi IWCt,s IWCt TWCm, 

IWCm

LWCm,2 LWCm,0.5

569 30.8 4.02 0.53 3.49 1.97  .38 0.18  .01 0.19  .01 

576^ 22.6 3.93 0.61 3.32 1.42  .22 0.08  .01 0.09  .01 

582 22.7 6.84 0.77 6.07 3.14  .29 0.23  .02 0.24  .02 

588 22.1 9.78 0.88 8.90 4.15  .41 0.49  .05 0.39  .02 

3.1.3 Mixed-Phase Results 

For the mixed-phase tests, both IWCi and supplemental LWCi were injected into the test section. 

Again, the TAT was maintained at a target temperature of –1 C to prevent the IWCi from melting. 

Evaporation was again calculated by measuring the humidity change in the test section with the cloud off 

and on. In this way, we set out to achieve a “known” mixed-phase cloud and examine its results on the 

SEA multi-wire probe. For the half-pipe, there is a contribution from both the IWC and LWC. For the 

cylindrical elements, the measured LWC in mixed phase would be that of the supplemental LWC plus a 

contribution due to a false response from ice particles. 

In this paper, we put forth and test the following hypothesis: 

The half-pipe TWC measurement in mixed-phase is a superposition of the individual IWC and 

LWC components. The individual LWC and IWC measurements can be corrected to the actual 

values using calibrations from separate LWC and IWC clouds. In mixed-phase, the LWC 

measurements should use the maximum measurement from either the 0.5 or 2.1-mm diameter 

element and subtract the false response due to the ice particles. 

The hypothesis is tested by solving for LWCt* and IWCt* (and hence TWCt*) as shown in 

Equation (10) and (11) using measurements made in mixed phase. Here, the asterisk (*) denotes 

values at the test section calculated using the mixed-phase hypothesis. These calculations require 

solving for the false response contribution of ice particles on the liquid-water elements, LWCFR, as 

shown in Equation (12). For this analysis, we use the average of the curve fit values (0.045  IWCt*) 
NASA/TM—2013-216571 11 

from Figure 5 to calculate LWCFR. However, the corrections for the specific element shown in Figure 5 

can be used. There are three unknowns in Equations (10) to (12), LWC

!, IWC!, and LWC"# , which are 

solved by iteration. 

LWC

! = max$LWC,%&', LWC,%*+-  LWC"# (10) 

IWC

! =

./012/3

!

%.5&6 (11) 

LWC"# = 0.045  IWC

! (12) 

To validate the hypothesis, we compare the calculated total water content injected, TWCi*, to the actual 

injected total water injected, TWCi. The TWCi* is calculated by adding the amount of humidity increase 

at the test section due to evaporation and sublimation with the cloud on compared to the cloud off—see 

Equation (7). If the TWCi* and TWCi are comparable then the proposed hypothesis of superposition is 

supported for the conditions tested. Note that the method only can validate the TWCt* values and cannot 

individually validate the IWCt* and LWCt* values because the individual contributions of evaporation 

and sublimation cannot be separated in the mixed-phase cloud. 

Figure 6 compares TWCi* and TWCi to examine the hypothesis put forth to interpret the SEA multiwire probe in mixed-phase conditions. For reference, a 1:1 line is included in the figure to facilitate 

comparison between the two values. The data used to calculate the TWCi* is shown in Table 3. For total 

water contents of 8.5 g/m3

 and less (data points 625, 646, and 636), the calculated injected TWCi* 

compares to within 6 percent of the actual injected TWCi. In two cases (646 and 636), the comparison is 

approximately 1 percent or less. For cases where TWCi is 10.7 g/m3

 and greater (673, 631, and 652), the 

TWCi* is less than the actual TWCi by 10 to 12 percent. This suggests that the hypothesis may be 

supported for total water contents below approximately 7.5 g/m3

 (e.g., TWCt* for data point 636) but 

seems to break down as the TWCt becomes larger. However, more data at the higher water loadings 

should be generated before reaching firm conclusions. Furthermore, these results are for velocities of 

85 m/s, 93 kPa, and the “small” particle size and the calibration curves, such as the factor 0.485 in 

Equation (11), may only be valid at those conditions. More data needs to be generated at other test 

conditions to confirm the validity of the hypothesis at those conditions. 

Figure 6.—Comparison of injected TWC calculated from measurements made in mixed phase to actual 

injected TWC for tests conducted at nominally 85 m/s and 93 kPa. This data tests the hypothesis that 

mixed-phase measurements superimpose on a non-interfering basis. 
NASA/TM—2013-216571 12 

TABLE 3.—DATA FROM MIXED-PHASE TESTS WITH ICE AND SUPPLEMENTAL LIQUID WATER 

[The test section velocity was nominally 85 m/s, the ambient pressure was 93 kPa, and the total air temperature was –1 C. 

Due to the below freezing temperature, the ice particles presumably would not melt and all contribution of LWC 

would be due to the supplemental water. A carrot (^) is included next to the data point number 

when the time period for the average values was adjusted from the last 100-sec.] 

Multi-wire measurements From algorithm Measured 

evap+sub 

Calculated 

injected 

Injected 

Data 

Point 

RH, 

% 

TWCm LWCm,2 LWCm,0.5 LWCt* IWCt* TWCt* TWCs+e TWCi* TWCi LWCi IWCi

625 25.5 2.280.23 0.470.01 0.480.01 0.30 4.08 4.38 0.80 5.18 4.90 0.88 4.02 

646^ 25.2 3.200.20 1.500.03 1.390.03 1.33 3.85 5.18 1.34 6.53 6.61 2.63 3.98 

636 25.9 4.250.24 1.510.04 1.060.02 1.23 6.23 7.46 1.02 8.48 8.52 1.71 6.81 

673 73.2 5.230.38 1.510.06 0.800.03 1.13 8.45 9.58 0.59 10.17 10.73 0.87 9.86 

631 25.5 4.590.34 1.120.04 0.620.02 0.77 7.89 8.65 1.00 9.65 10.78 0.87 9.91 

652 26.9 5.620.39 2.070.13 1.120.05 1.71 8.07 9.78 1.27 11.05 12.56 2.63 9.93 

Figure 7.—Images of the front face of the SEA multi-wire probe taken during the March 2012 test campaign. 

The images are from data point 582 which had a test section IWCt of ~6 g/m3

. With the ice cloud on, the 

half-pipe of the SEA multi-wire probe has what appear to be dry and wetted regions. The inset view shows 

an image of the probe during the same data point but backlit producing a shadow view of the elements. 

3.1.4 Multi-Wire Probe Imaging 

As part of the March 2012 test campaign, a high-definition video camera was positioned to image the 

elements of the SEA multi-wire probe with the cloud on. This was done to see if a portion of the 

incoming water flux was either bouncing or splashing from the elements prior to evaporation. In all of the 

tests with injected IWCi, a portion of the half-pipe appeared to be wetted and a portion appeared to be 

running dry. Figure 7 shows two still images extracted from the videos for data point 582: one image was 

taken with the cloud off, the other with the cloud on. In this case, the test section IWCt was calculated to 

be ~6 g/m3

 and the element was wetted from approximately the mid-point to the rightmost visible portion 

of the element. A cursory examination of other videos suggests that the wetted region increases with 

increasing IWCt. Videos taken during LWC-only data points did not have visible dry and wetted regions. 

Finally, the probe was back illuminated during portions of the video recording to see if there was any 

evidence of splashing. The inset image in Figure 7 shows a portion of a frame from data point 582 that 

captured an event where beads of water appear to be run out of the half-pipe. A more detailed analysis is 

currently underway as well as discussions with the manufacturer of the probe to help interpret these 

observations from the video data. 

Cloud Off IWC Cloud On 

Dry Wet

Backlit view 

Run out?
NASA/TM—2013-216571 13 

3.2 Cloud Uniformity 

The uniformity of LWC and IWC clouds was gauged non-intrusively using a laser sheet positioned in 

the test section normal to the flow direction (Figure 8). As water and ice particles pass through the laser 

sheet, they scatter light, which is then measured by a camera. The laser sheet was created using a fibercoupled diode laser (1.5 W, 532 nm) and spanned the entire height of the test section but only half the 

width limited to the existing window size. The laser sheet was positioned approximately 70 mm ahead of 

the SEA multi-wire probe. A scientific-grade slow-scan imaging camera was positioned downstream and 

outside of the test section looking back at the laser sheet at an off-normal viewing angle through the large 

side window in the Cascade rig. In this position, the camera would capture forward-scattered light from 

the particles passing through the laser sheet. The camera took a sequence of images (approximately 50 

images over a period of ~25 sec) beginning roughly as the cloud was turned on. Each image had an 

exposure of 0.5 sec and had a 14-bit dynamic range. During the imaging, the light in the test cell was 

minimized because any light source could be detected as a false signal by the imager. Furthermore, 

reflections of the laser source itself from surfaces in the test cell could mask any signal from particlescattered light. 

The right-hand side of Figure 8 shows a sample image taken during testing with an ice-only cloud 

with a TAT < 0 °C. The yellow line in this figure defines a region of interest (ROI) used for the 

subsequent analysis. This ROI was defined to avoid areas of laser-light reflection (seen as bright spots 

near the periphery of the ROI) which masks the forward-scattered light from the particles. The average 

intensity in the ROI was computed for each image of the sequence. Results from the LWC-only data 

points are presented in Figure 9. In the left graph of this figure, the average intensity of each image is 

shown as a function of time. The start of the imaging sequence was manually initiated to coincide with 

the beginning of the spray. However, only one data point (641) shows an increasing trend near the 

beginning of the sample period while the other data points appear to be about constant suggesting that the 

spray was initiated prior to the beginning of the imaging sequence. On the right graph shown in Figure 9, 

the intensity averaged over all the image frames (or as denoted in the figure) is plotted against the liquid 

water content calculated to be at the test section, LWCt (injected LWCi minus evaporated LWCe). The 

graph shows that the average intensity appears to increase proportionally with increasing LWCt. Figure 10 

shows results from a similar analysis performed with an IWC-only cloud. A larger average intensity is 

Figure 8.—Laser sheet cloud uniformity setup and sample image. 

Test Section 

CW Laser – diode – fiber coupled 

Sheet generating optics 

203 mm cylindrical lens

Window 

(63.5 mm width)

132 mm

254 mm

Camera

Sample Image

ROI 

Window
NASA/TM—2013-216571 14 

Figure 9.—Mean intensity level for the region of interest shown in Figure 8 with LWC-only 

sprays at 85 m/s and 93 kPa total pressure. The left graph shows the mean intensity for 

each image as a function of time. The right graph shows a time-average intensity across 

all image (or as specified) plotted versus LWCt. 

Figure 10.—Mean intensity level for the region of interest shown in Figure 8 with IWC-only 

sprays at 85 m/s and 93 kPa total pressure. The left graph shows the mean intensity for 

each image as a function of time. The right graph shows the time-averaged intensity 

plotted versus IWCt. 

measured with the IWC clouds presumably owing to the increased concentration. Comparison of a linear 

curve fit of the average intensity versus LWCt shows a slope of 2200 ± 364 counts per g/m3

. Here the 

error band is 95 percent confidence interval. The slope of the average intensity versus IWCt is 2362 ± 155 

counts per g/m3

 and is essentially identical considering the confidence intervals of the data. This result 

suggests that the measured intensity is a good first-order measurement of concentration, independent of 

the water phase. 

Figure 11 shows images generated by averaging the entire image sequence and applying a color lookup table which maps an average-intensity value to a color. The pixel intensities range from 2000 to over 

30,000 counts. The images were transformed to map the pixel intensities to the 2D plane of the laser 

sheet. As a result, the image corresponding to data point 576 appears slightly warped as is evident by 

inspecting the upper portion of this image. The upper region of the remaining images was cropped and 

scaled to match the left-most image. Note that only the middle portion of the test section is mapped in this 

analysis—regions without information are shown in black. 
NASA/TM—2013-216571 15 

Figure 11.—Cloud uniformity estimates using laser sheet measurements averaged over the sampling period 

for 3 values of IWCt at the test section for the same flow condition (nominally 85 m/s and 93 kPa). 

Figure 11 shows 3 data points from the 85 m/s and 93 kPa data set with increasing IWCt from left to 

right. The intensity profiles show ellipse-like concentric regions with the highest counts near the center of 

the test section for each case. As the IWCt increases, greater intensities and steeper intensity gradients are 

observed in the test section. In previous work (Ref. 14), the intensity has been related to the local LWC or 

IWC through the use of a transfer-point measurement via an in-situ calibration (usually via a single-point 

probe measurement) to produce quantitative LWC or IWC information across the test section. In this 

work, we are examining the use of the average intensity and bulk (i.e., assumed uniformly mixed) LWCt

or IWCt as the transfer function. For example, one could use data from the curve fit from the data shown 

in Figure 10. However, such a transfer function needs also to consider the local flow velocity where 

increases in velocity also presumably increase the measured intensity counts as more particles traverse a 

given point in a given time interval. It is noted that this work is still preliminary and further analysis is ongoing. Ideally, future tests could map the entire test section which would require a laser sheet to span the 

entire cross-section. Ultimately, such data could be used to calculate local IWC and LWC values which 

then could be used in analyses like that performed in the previous section of the SEA multi-wire probe 

analysis 

3.3 Particle Size-Spectra and Velocity 

Due to the small test-section size of the Cascade rig in the NRC RATFac, flight-probe technology has 

not been easily adaptable to measure particle size spectra and velocity. As a result, NRC has been 

developing a custom Particle Imaging Velocimetry (PIV) system using a shadowgraph method (Ref. 7) 

for the purpose of measuring particle size and velocity. For the 2012 test entry, the PIV system was used 

to image particles in the center of the test section near where the leading edge of airfoil would be (see 

Figure 13 or refer back to Figure 1). This location is approximately 150 mm downstream of where the 

SEA multi-wire probe was located. For testing with the NRC PIV system, the test section was empty and 

the SEA multi-wire probe was retracted. 

30103

23 

16 

9 

Pixel 

Intensity 

576 

IWCt = 3.3 g/m3

583 

IWCt = 6.1 g/m3 588 

IWCt = 8.9 g/m3

132 mm ~63.5 mm 
NASA/TM—2013-216571 16 

Additionally, NASA is investigating the use of a high-speed camera coupled with a pulsed LED 

backlight to measure particle size and velocity. For this 2012 test entry, the high-speed system was used 

to take measurements near the ice-injection duct exit (Figure 14). Various positions near the ice injection 

outlet were measured—during the first day of tests it was observed that there were significantly fewer 

particles at the centerline of the ice-injection duct so the camera was repositioned to focus on regions 

within ~ 10 mm of the inner periphery of the duct. Additionally, some measurements were made near the 

bellmouth entrance (see Figure 1 for the bellmouth location). 

A quick comparison of both the PIV and high-speed camera system is shown in Table 4. Each 

measurement is essentially a point measurement in the cloud due to the small field of view (FOV). By 

making particle size measurements at two different downstream locations in the cloud, a desired objective 

was to quantify any changes in particle size and perhaps particle state, the latter would be accomplished 

by contrasting particle shape descriptors. Particle velocity measurements help quantify the flowfield and 

particle impact dynamics. The status of the NRC PIV and NASA high-speed imaging is presented below. 

TABLE 4.—QUICK COMPARISON OF PARTICLE IMAGING SYSTEMS 

System PIV High-speed camera / 

LED 

High-speed camera / 

LED 

Mode N/A Particle Counting Velocity Measurement 

Field of view 4.5- by 4.5-mm 9.07- by 5.67-mm 

(1280800 pixels) 

9.07- by 1.08-mm 

(1280152 pixels) 

Resolution 1.3 m/pixel 7.08 m/pixel 7.08 m/pixel 

Exposure time 9 ns 100 to 300 ns 100 to 300 ns 

Sample rate 2.5 to 5 fps 1000 fps 30,000 fps 

3.4 NRC PIV 

The system used by NRC for particle imaging was a non-intrusive, high magnification shadowgraphy 

technique based on a LaVision PIV system. A diffused laser beam of uniform intensity was pulsed into a 

high resolution camera through a long distance microscope. The pulsed backlight illumination freezes the 

particles in a sample volume defined by the field of view and depth of field of the optics. In addition to 

particle sizing, this system can also be used for particle velocity and direction measurements. 

One of the main advantages of this system is the short exposure time due to the laser pulse duration of 

9 ns. This freezes the particles in the image thus minimizing the blur due to movement even at high test 

speeds of 200 m/s and beyond. Typically, an image intensity is on the order of 3000 counts in clean flow 

and approximately 500 counts when a particle was imaged. The system also has high resolution which 

provides the ability to resolve a wide range of particle sizes, approximately 10 to 1500 m in diameter. This 

high resolution combined with the field of view requires large amounts of data to be transmitted from the 

camera to the data storage computer. The result is a relatively slow sample rate thereby requiring test times 

in the order of 10 to 20 min depending on particle concentration and distribution characteristics. The general 

specifications of this shadowgraphy system are given in Table 4 with a detailed description of its use for ice 

particle imaging and particle sizing provided elsewhere (Refs. 7 and15). For this work, the NRC 

shadowgraphy PIV system was oriented normal to the axis of the test section, with the focal plane and line 

of site coincident with the tunnel centerline as shown in Figure 13. Due to the low pressure and cold 

environment in the test chamber, the imaging hardware was installed in an enclosure which maintained the 

temperature above 0 °C and pressure above 70 kPa for all test conditions. To eliminate the need for a similar 

enclosure for the laser and its power supplies, these items were installed outside of the chamber with the 

laser light entering the chamber through a window. This laser beam was then directed to the diffuser using 

mirrors. The PIV computer was located outside of the chamber and was used to control the system and store 

the images. Figure 12 is a cropped image, approximately 1⁄4 of full size, and illustrates the type of particle 

imaging that was obtained and used for particle analysis. Post processing was conducted using LaVision 

DaVis 8.1 particle sizing/image analysis software with the results to be presented at a later date. 
NASA/TM—2013-216571 17 

Figure 12.—Sample particle 

images from the NRC 

shadowgraphy PIV system. 

Figure 13.—Setup of NRC shadowgraphy PIV system used for particle 

imaging in the test section 

3.4.1 NASA Phantom Camera/LED System 

To measure both particle size distributions and velocity, NASA conducted some exploratory work 

using a high-speed camera coupled with a pulsed LED backlight system as seen in Figure 14. To avoid 

blurring of particles, it is essential to have an exposure which is shorter than the time required for a 

particle to move one (ideally) or just a few pixels. The camera’s minimum exposure time is 1 μs. At 

85 m/s, it only takes ~83 ns to move one pixel (given a 7 m per pixel resolution). During 1 μs, a particle 

would have moved 12 pixels to cause blurring. To help minimize blurring, a high-current short-duration 

pulsed LED was triggered at a 700 to 900 ns delay after the camera shutter was open. This reduced the 

effective exposure time to values of 300 to 100 ns, respectively. At 300 ns, the LED provided sufficient 

light to get good contrast between the particles and background. In 300 ns, particles moving at 85 m/s 

would have moved ~3 pixels (or ~ 21 m)—this distance would be a bias measurement error in the 

horizontal direction. Data was also taken using a 100 ns delay, which offers less bias error, but the images 

were darker and consequently had less contrast. 

1 mm 

Camera and 

Optics Inside 

Enclosure

Laser 

Diffuser 

Air, Ice and Droplet 

Flow Direction 
NASA/TM—2013-216571 18 

Figure 14.—High-speed camera and pulsed LED backlight taking particle images during testing (left) 

and determining the imaged location (focus region) using a laser and graduated glass (right). 

The high-speed camera was operated in one of two modes: a particle-counting or particle-velocitymeasurement mode. For particle counting, the objective was to gather a large number of independent 

images at the same physical location in space ensuring that no particles were counted more than once. In 

this mode, the camera’s full imager (1280800 pixels) was utilized with images taken at 1000 fps. Optics 

resulting in a resolution of ~7 m per pixel were used for this initial testing, although a finer resolution is 

achievable with different optical components. More specifications for the system in particle counting 

mode are shown in Table 4. At the lowest velocities of 85 m/s, a particle traverses a horizontal distance 

of 85 mm in 0.001 sec, well in excess of the horizontal FOV of 9 mm, thus ensuring that particles are 

only counted once in the particle counting mode. A sample image taken while operating the camera in 

particle-counting mode is shown in Figure 15. The image has been cropped from the original size to 

814542 pixels for clarity. 

An algorithm is currently under development to discriminate between imaged particles that are within the 

depth of field (i.e., in focus) and those that are outside the depth of field (i.e., out of focus). The algorithm uses 

an edge-detection filter to identify sharp gradients in the image. A sharp gradient corresponds to an in-focus 

particle. Currently, various criteria relating the gradient to the degree of focus are being examined. For one 

such criterion, the resulting in-focus particles are shown in Figure 16 where the perimeter of the in-focus 

particles is colored in red. Here, several steps are used to identify the in-focus particles including specifying a 

gradient value and filtering out particles below a certain size. A minimum-detectable particle size, which may 

be a number like 4 to 6 pixels across a particle, results from the requirement to identify a gradient on both sides 

of a particle (e.g., 2 to 3 pixels for each edge). At 7-μm per pixel, the minimum detectable particle size using 

the current optics would be 28 to 42 μm in diameter. The resulting dataset of particle sizes is being further 

examined with the goal of calculating statistics on the distribution (i.e., DV10, DV50, and DV90). A factor 

affecting those statistics include understanding the relationship between the particle size and depth of field 

which is needed to calculate the particle concentrations—work is ongoing to develop those relationships for 

the high-speed camera/LED system. 

In particle-velocity-measurement mode, the objective was to capture the same particle in at least 2 

consecutive frames so that the distance and hence the velocity of the particle could be measured given the 

known duration between frames. At a resolution of 7 m per pixel, a particle travelling at 85 m/s will 

traverse the entire horizontal FOV in just over 100 μs. During that period, a minimum of 2 frames must 

be imaged which corresponds to a frame rate of just under 20,000 fps. For the March 2012 testing, the 

camera was operated at ~30,000 fps which allowed measuring particle velocity at the highest test velocity 

of 135 m/s. The 30,000 fps was also near the upper limit of the LED pulsing system. At these higher 

framing rates, only a portion of the imager could be used due to image transfer-time limitations of the 

camera. It was desired to preserve the horizontal FOV to maximize the potential to image moving 

particles which traverse in the horizontal plane. Thus, the vertical FOV needed to be cropped to 152 

pixels to allow the needed throughput of images at the highest frame rate. The specifications for the 

system in particle-velocity-measurement mode are summarized in Table 4. 

LED 

Camera

Equation 
NASA/TM—2013-216571 19 

Figure 15.—Sample image of particles taken using the particle counting mode.

Figure 16.—Particles determined to be in-focus using a developmental algorithm. 

Original Image 

Analyzed Particles

2

4 3

5

6 7

8 9

10 11

12

1
NASA/TM—2013-216571 20 

Figure 17.—Images from two consecutive frames captured using a high-speed camera set to capture particle 

velocity. The time interval between the top and bottom image is of ~33 μs and the coordinates show the centroid of 

a particle (in pixels) which moves between the frames. 

Figure 17 shows two consecutive frames taken using the particle-velocity-measurement mode. For 

this test, the camera was sampling at 30010 fps resulting in approximately 33 μs between images. The 

centroid of one particle (in pixel coordinates) is shown in both of the images. That particle traversed a 

horizontal distance of 432 pixels (3.06 mm) and a vertical distance of 33 pixels (0.234 mm). This results 

in velocities of 91.8 and 7.0 m/s in the horizontal and vertical directions, respectively. For this particular 

test (data point 867), the average air velocity coming from the ice-injection duct was measured to be 84.1 

 0.5 m/s with the cloud-off (the error bars represent the standard deviation of the measurements taken 

during a 30-sec sample period). Such results could be compared to the cold-jet velocity or perhaps to get 

local velocity measurements. Additionally, this system could be used to image particles just prior to and 

during impact onto an icing test article. 

4.0 Conclusions

In this paper, we describe some cloud-characterization work performed at the NRC RATFac during 

March and April 2012 in preparation for future scaling studies. The cloud characterization included water 

content, cloud uniformity, and particle size-spectra measurements. Water content measurements were 

made using a SEA multi-wire probe. Cloud uniformity measurements came from light scattered from 

particles passing through a laser sheet normal to the flow. Finally, particle size measurements were made 

using two developmental shadowgraph systems. 

A series of tests examined the SEA multi-wire probe measurements in pure LWC and IWC sprays as 

well as combined IWC and supplemental LWC sprays, the latter to simulate a “known” mixed phase. 

LWC-only measurements suggest that the test section liquid water content, LWCt, can be deduced using 

the inject water flow rates and measurements of the amount of water that has evaporated. Overall, there is 

good agreement with the measured LWCm values with LWCt for all three elements of the SEA multi-wire 

probe. Applying a similar methodology to an IWC-only cloud showed that the half-pipe element of the 

SEA multi-wire probe measures approximately half (~48.5 percent) of the IWC at the test section for a 

tunnel velocity of 85 m/s and total pressure of 93 kPa. Image data of the front face of the SEA multi-wire 

probe during testing suggests that not all of the IWC is being evaporated by the half-pipe before being 

swept away downstream. In addition, data indicates a false response of approximately 4 to 5 percent on 

the cylindrical hot-wire elements to the fully-glaciated clouds generated in this study. 

A hypothesis and analysis algorithm was proposed for interpretation of mixed-phase results using the 

SEA multi-wire probe which infers the independent LWCt* and IWCt* (and hence TWCt*) using direct 

measurements from the probe. The hypothesis asserts that the LWCm and IWCm measurements 

superimpose on a non-interference basis allowing use of calibrations from the liquid and ice-only tests. 

The results were validated by calculating an injected value from TWCt* and adding the sublimated and/or 

evaporated vapor measured by an independent sensor. Note that the individual IWCt* and LWCt* cannot 

be independently confirmed because we cannot differentiate contributions of sublimation and evaporation 

to the total vapor increase measured in the test section during mixed phase. 

(276, 80)

(708, 47) 
NASA/TM—2013-216571 21 

Results from the presented data set (85 m/s, 93 kPa, “small” particle size grinder setting) support the 

hypothesis for values of TWCt less than 7.5 g/m3

 but results begin to diverge above this value. The 

diverging results at larger TWC may suggest that the superposition assumption breaks down once a highenough water loading is achieved. Further testing is required to assess the probe’s response at higher 

water loadings. In addition, other factors may explain some of the disagreement including particle size 

variations with ambient humidity or test-section non uniformities, neither of which are yet well 

understood and may affect the calibration curves obtained during single-phase testing. Moreover, only a 

limited set of data (85 m/s, 93 kPa, “small” particle size), have been explored and additional testing is 

required to confirm the hypothesis. Finally, the current results use IWC plus supplemental LWC and 

confirmation of the proposed hypothesis and mixed-phase algorithm still require testing using a mixedphase resulting from melting ice-crystal clouds which is the desired scenario for engine icing. 

To supplement existing SEA multi-wire traverse data, the uniformity of LWC and IWC clouds were 

gauged non-intrusively by measuring the scattered light of particles passing through a laser sheet 

positioned in the test section normal to the flow direction. Plots of the image-average intensity versus 

LWCt and IWCt suggest that the measured intensity is a good first-order measurement of concentration, 

independent of the water phase. Additionally, images of the average intensity across about 1⁄2 the test 

section helped visualize the IWC distribution in the test section which showed ellipse-like concentric 

regions of increasing intensity with the highest counts near the center of the test section for each case. As 

the IWCt increases, greater intensities and steeper intensity gradients are observed in the test section. It is 

noted that this work is still preliminary and further analysis is on-going. 

Two different shadowgraph methods to measure particle size and velocity were used during the 2012 

test entry. The first utilized a Particle Imaging Velocimetry (PIV) system and imaged particles in the 

center of the test section just upstream of where the leading edge of the model would be located (tests 

were conducted with an empty test section). The second utilized a high-speed camera coupled with a 

pulsed LED backlight that helped to reduce motion blur by reducing the effective exposure time. Both 

methods have generated promising image sets that will be processed for size statistics (i.e., DV10, DV50, 

and DV90). A further goal is to quantify any changes in particle size and perhaps state by making particle 

size measurements at two different downstream locations in the cloud. 

The PIV shadowgraphy system was effective in providing good quality images of particles traveling 

through the test section of the rig and it is these images that will be used to establish particle size 

distributions and morphology characteristics. The high-speed camera system was operated in one of two 

modes: one for particle-counting and the other for velocity measurements. An algorithm is currently being 

developed to discriminate between imaged particles that are within the depth of field (i.e., in focus) and 

those that are outside the depth of field (i.e., out of focus). The algorithm uses an edge-detection filter 

which identifies in-focus particles as those with sufficiently sharp gradients. The system is currently able 

to resolve particles down to ~28 μm but could potential resolve smaller particles with different optics. By 

making two sets of measurements at different downstream locations offers the possibility to discriminate 

particle phase. 

Characterization of the environmental conditions as well as their influence on ice-crystal icing is 

critical to understanding engine power-loss events. This work, along with previous work both by this 

group and others, has begun to make strides in understanding the complex environmental conditions 

leading to ice-crystal icing. However, further work is required to continue cloud characterization in icecrystal and mixed-phase environments. This includes further interpretation of existing results and 

additional experimental data. The results from this work will help guide future work including scaling 

studies. 

References 

1. Struk, P., Currie, T., Wright, W. B., Knezevici, D. C., Fuleki, D., Broeren, A., Vargas, M., and Tsao, 

J. “Fundamental Ice Crystal Accretion Physics Studies,” SAE 2011 International Conference on 

Aircraft and Engine Icing and Ground Deicing, 2011-38-0018 or NASA/TM—2012-217429, 2012. 
NASA/TM—2013-216571 22 

2. Currie, T. C., Struk, P. M., Tsao, J., Fuleki, D., and Knezevici, D. C. “Fundamental Study of MixedPhase Icing with Application to Ice Crystal Accretion in Aircraft Jet Engines,” 4th Atmospheric and 

Space Environments Conference, AIAA 2012-3035, 2012. 

3. Anderson, D. N. “Manual of Scaling Methods,” NASA/CR—2004-212875, 2004. 

4. Currie, T. C. “Altitude Scaling of Ice Crystal Accretion,” 5th AIAA Atmospheric and Space 

Environments Conference, American Institute of Aeronautics and Astronautics, Reston, VA 

(submitted for publication). 

5. Mason, J. G., Strapp, J. W., and Chow, P. “The Ice Particle Threat to Engines in Flight,” 44th AIAA 

Aerospace Sciences Meeting and Exhibit, AIAA-2006-206, 2006. 

6. Bencic, T. “Development of Advanced Optical Instrumentation for use in the NASA Glenn Icing 

Research Tunnel,” 39th Aerospace Sciences Meeting and Exhibit, AIAA-2001-396, 2001. 

7. Galeote, B. “Ice Crystal Particle Measurement Using Shadowgraph Imaging Techniques,” AIAA 

Atmospheric and Space Environments Conference, AIAA 2010-7531, 2010. 

8. Knezevici, D. C., Fuleki, D., Currie, T. C., and MacLeod, J. D. “Particle Size Effects on Ice Crystal 

Accretion,” 4th AIAA Atmospheric and Space Environments Conference, AIAA 2012-3039, 2012. 

9. Lilie, L., Emery, E., Strapp, J. W., and Emery, J. “A Multiwire Hot-Wire Device for Measurment of 

Icing Severity, Total Water Content, Liquid Water Content, and Droplet Diameter,” 43rd AIAA 

Aerospace Sciences Meeting and Exhibit, AIAA-2005-859, 2005. 

10. Van Zante, J. F., Ide, R., and Steen, L.-C. “NASA Glenn Icing Research Tunnel: 2012 Cloud 

Calibration Procedure and Results,” 4th AIAA Atmospheric and Space Environments Conference, 

AIAA-2012-2933, 2012. 

11. Emery, E., Miller, D., Plaskon, S. R., Strapp, J. W., and Lilie, L. “Ice Particle Impact on Cloud 

Water Content Instrumentation,” 42nd Aerospace Sciences Meeting and Exhibit, AIAA–2004–0731 

and NASA/TM—2004-212964, 2004. 

12. Van Zante, J. F., Addy, H. E., and Ide, R. F. “PSL/Cox Technology Demonstration II, Dec. 2011 

Test Summary,” NASA Glenn Research Center, Cleveland, OH, 2012 (unpublished). 

13. Knezevici, D. C., Fuleki, D., and MacLeod, J. “Development and Commissioning of the Linear 

Compressor Cascade Rig for Ice Crystal Research,” International Conference on Aircraft and Engine 

Icing and Ground Deicing, 2011-38-0079, 2011. 

14. Papadakis, M., Hung, K. E., Vu, G. T., Yeong, H. W., Bidwell, C. S., Breer, M. D., and Bencic, T. J. 

“Experimental Investigation of Water Droplet Impingement on Airfoils, Finite Wings, and an S-Duct 

Engine Inlet,” NASA/TM—2002-211700, 2002. 

15. Knezevici, D. C., Fuleki, D., Currie, T., Galeote, B., and Chalmers, J. “Particle Size Effects on Ice 

Crystal Accretion – Part II,” 5th AIAA Atmospheric and Space Environments Conference, American 

Institute of Aeronautics and Astronautics, Reston, VA (submitted for publication).